Sulfide Oxidation by 2,6-Bis[hydroxyl(methyl)amino]-4-morpholino-1,3,5-triazinatodioxomolybdenum(VI): Mechanistic Implications with DFT Calculations for a New Class of Molybdenum(VI) Complex

Sulfide oxidation is accomplished by a new class of dioxomolybdenum(VI) catalyst (1) that uses the tridentate 2,6-bis[hydroryl(methyl)amino]-4-morpholino-1,3,5-triazine ligand to form a five-coordinate molybdenum(VI) center. Resonance Raman spectra show that the dioxo groups on the Mo(VI) oxygens readily exchange with water in an acetonitrile media that allows O-18 labeling of catalyst 1. The model oxidation reaction was the conversion of thioanisole (2) to the corresponding sulfoxide with 4% of 1 using an equimolar amount of H2O2. in MeCN-d(3). Oxygen-18 labeling experiments with either O-18-labeled 1 or O-18-labeled H2O2 are consistent with a sulfide oxygenation pathway that uses a eta(1)-Mo(OOH) hydroxoperoxyl species (3). The hypothesized intermediate 3 is initially formed in a proton transfer reaction between 1 and H2O2. Oxidation is hypothesized via nucleophilic attack of the sulfide on 3 that is supported from a Hammett linear free-energy relationship for para-derivatives of 2. A Hammett reactivity constant (rho) of -1.2 +/- 0.2 was obtained, which is consistent with other rho values found in prior sulfide oxidation reactions by group 6 complexes. An Eyring plot of the 2 oxidation by 1 gives an E-a of 63.0 +/- 5.2 kJ/mol, which is slightly higher than that of a similar oxidation of 2 by the molybdenum(VI) complex, oxodiperoxo (pyridine-2-carboxylato)molybdate(VI) bis(pyridine-2-carboxylic acid) monohydrate (5). Computational modeling with density functional theory (DFT) of the complete reaction profile gave enthalpy and entropy of activations (64 kJ/mol and -120 J/mol.K, respectively) within 1 standard deviation of the experimental values, further supporting the hypothesized mechanism.

Automated summary

A new class of dioxomolybdenum(VI) catalyst is used to achieve sulfide oxidation, with experiments and computational modeling supporting a hypothesized reaction mechanism.